summation of results in mysql and then dividing the result by 2

Again, to continue with my mysql noob questions :) I want the following to be just one result:

select sum(sales_today) from table1
union
select sum(sales_yesterday) from table1

So right now that gives me two results, how do I make it give me only one, the summation of the two results? And how开发者_C百科 can I do to apply further equations to the result, such as dividing it by 2?

Thank you!

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SELECT (SUM(sales_today) + SUM(sales_yesterday)) / 2 FROM table1

For more details on what you can use, see MySQL Functions and Operators.

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You can add Expressions like you add fields. For easier access you should use the "AS"-keyword to give meaningful names:

select sum(sales_today) as TodaysSales,
    sum(sales_yesterday) as YesterdaysSales,
    ((sum(sales_today) + sum(sales_yesterday)) / 2) as TwoDayAverage 
from table1